A use of mobile communications networks has tremendously increased over the last decade. Operators of mobile communications networks have increased a number of base stations in order to meet an increased request for service by users of the mobile communications network. The base stations typically comprise radio systems for relaying radio signals. The radio signals are typically relayed into a cell of the mobile communications network. It is of interest for the operator of the mobile communications network to reduce the running costs of the base stations. It is one option to implement the radio system as an antenna embedded radio system. With the antenna embedded radio system some of the hardware components of the radio system may be implemented on a chip. The antenna embedded radio system therefore reduces the costs of the base station. Implementing the radio system as the antenna embedded radio system reduces space needed to house the hardware components of the base station. Power consumption during normal operation of the radio system is substantially reduced when implementing the antenna embedded radio system comprising the chip.
It is of interest to provide a reliable quality of service to an individual user of the mobile communications network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. One of the several techniques comprises beam forming capabilities in order to direct a beam relayed by the radio system in different directions to improve service coverage within the cells of the mobile communications network. The beam forming techniques rely on defined phase and amplitude relations between several of the antenna elements of the active antenna system. Calibration of transmit paths and receive paths is required to provide the defined phase and amplitude relationship between the beams. The calibration allows the estimation of a phase and amplitude deviation accumulated along the transmit path of the radio system. Likewise the calibration comprises estimating phase and amplitude deviations accumulated along the receive paths of the radio system. The calibration may further comprise a determination of transit times needed for a message signal to travel from the digital radio interface to the antenna element in order to be relayed. In a second step the phase and amplitude deviation accumulated along the transmit paths can be corrected. An appropriate phase and amplitude change may be applied to the individual transmit paths to yield the defined phase and amplitude relationship between the individual transmit paths of the radio system, in order to allow for beam forming techniques.
In a modern mobile communications network a payload signal is provided as a packetized payload signal to the radio system. Different to a continuous payload signal the packetized payload signals have a defined temporal order when the packetized payload signal is provided to a digital radio interface. Within the radio system some processing may be applied to the packetized payload signal. The processing may comprise the packetized payload signal passing several buffers and phase locked loops (PLLs). With the data processing the defined temporal order of the packetized payload signal may be deteriorated or even destroyed. In the prior art it was common practise to calibrate transmit paths along which the packetized payload signal travels when being relayed by the radio station. The relaying by the radio station comprises the data processing. The present invention provides a calibration of the transmit paths and a calibration of the digital predistortions when relaying packetized radio signals. Therefore the present invention is adapted to ascertain the temporal order of the packetized payload signal even with several steps of digital data processing applied to the a packetized payload signal. The present invention discloses the calibration of the phase and amplitude changes and the updating of the digital predistortion in the context of packetized internal radio signals of the system. It is to be understood that a co-pending application of the applicant discloses the calibration of phase and amplitude changes and the updating of the digital predistortion in the case of a non-packetized internal radio signals (U.S. application Ser. No. 12/416,596, now issued as U.S. Pat. No. 8,243,851) which is incorporated herein by reference.
Applying the phase and amplitude changes to the transmit paths of the radio system strongly relies on transfer characteristics of the radio system being linear. Typically, an amplifier used within the transmit paths causes non-linearities within the transfer characteristics of the transmit paths. Analogue predistortion or digital predistortion are known methods for correcting the non-linearities of the transmit paths. It is of interest to provide the digital predistortion prior to the applying of the phase and amplitude changes. With significant non-linearities in the transfer characteristics of the transfer paths, the phase and amplitude changes will not yield the defined relative phase and amplitude relationship needed for the beam forming techniques.
The calibration of the phase and amplitude changes and the digital predistortion require a feedback path. The feedback path is in both cases used in order to evaluate any changes a radio signal undergoes when being relayed along the transmit paths. This holds for both a calibration signal as well as the payload signal being relayed by the radio system.
The prior art discloses two distinct feedback paths for calibrating the phase and amplitude changes and the digital predistortion. This requires time and it would be advantageous to calibrate the radio system faster and more efficiently. The two feedback paths are expensive to implement.